Foamed articles have represented one of the fastest growing markets in the polymer industry in the past 15 years. This growth is expected to continue, and it is claimed that the potential usage of foamed polymers could far exceed the ability of the polymer industry to supply the needed materials.
The market for flexible foams is currently dominated by chemically crosslinked polymers, for example, polyurethanes and sponge rubbers. The time required for curing these materials is costly; for example, vulcanized elastomer foams such as vulcanized EPDM can require as long as two hours or more to cure. Also, the resultant lack of reprocessability can be a major handicap since many foaming operatings generate up to 25% scrap.
The present invention relates to unique and novel process for forming elastomer foams which have been found to possess several unexpected and useful characteristics. These novel foams contain a low concentration of sulfonate groups which are covalently appended to the elastomeric polymer backbone. These flexible ionomer foams are effectively crosslinked by the physical associations of sulfonate salt groups from different molecules. It has been found that in specified sulfonated elastomer systems these associations can provide a strong and tough material at use temperatures; yet at sufficiently elevated temperatures these associations can be disrupted to enable melt flow and melt fabrication. An important aspect of this material as specified in this invention is that it contains a nonvolatile polar plasticizer which greatly improves the flow or processability of the material at processing temperature. The continued presence of the nonvolatile polar plasticizer in the foam subsequent to the foaming process is crucial for many of the important characteristics of these novel foams such as the thermal reformability, and the melt reprocessability. A major advantage in the production of elastomer lightly sulfonated thermoelastic foams is that no post cure is required. This can result in a large savings in production time, machinery and cost. Also, no precure is required, and this can also make the foaming process less complicated and more efficient. These process advantages together with the virtue of melt reprocessability, thermal reformability, and the temperature insensitivity of the material during the foaming process make these foams extremely attractive candidates for a variety of flexible foam applications.
The preferred sulfonated elastomeric compositions of the present invention are derived from elastomeric polymers having a primarily hydrocarbon backbone with olefinic unsaturation, especially elastomeric polymers such as butyl and EPDM rubbers. Some processes for sulfonating such polymers are described in U.S. Pat. Nos. 3,642,728 and 3,836,511. Some methods for using polar additives to weaken the physical crosslinks at elevated temperature and improve processability in non-foamed articles of these lightly sulfonated polymers are included in U.S. Pat. No. 3,847,854.
U.S. Pat. No. 3,867,319 teaches a process for foaming an ionic polymer using a volatile polar plasticizer. A preferred method of foaming taught by that patent is dissolving of the ionic polymer in a solvent containing a volatile polar plasticizer and foaming by heating at atmospheric pressure in an oven. That patent differs from the instant invention in that a nonvolatile polar plasticizer which is a high temperature flow improver is specified in the instant invention and, as mentioned above, the nonvolatile polar plasticizer is required to obtain the unique set of characteristics of the flexible foams of the instant invention. In addition, the preferred method of foaming taught in U.S. Pat. No. 3,867,319 (which is described above) was found to be ineffective for producing satisfactory foams of the ionic elastomer materials considered here.
U.S. Pat. No. 3,947,387 claims novel ionic foams which are produced by using a volatile polar plasticizer. Again, such foams lack the essential ingredient of the non-volatile polar plasticizer which is critical to several of the most important characteristics of the foam of the instant invention.
U.S. Application Ser. No. 199,799, filed Nov. 17, 1971 and now U.S. Pat. No. 4,053,548, describes a fabrication process for multiphased plastics which includes ionic plastic foams. That process deals only with ionomers derived from a plastic material and explicitly excludes elastomeric polymers;
therefore this application does not bear on the instant invention which is concerned specifically with the novel characteristics of an ionic elastomer foam.
The present invention differs from the ionic polymer foams disclosed in U.S. Pat. No. 3,322,734, herein incorporated by reference, in that the products of the present invention are preferably neutralized completely. The strong association of these fully neutralized ionic polymers of the present inventions are weakened at elevated temperatures by the addition of suitable plasticizing agents which disrupt the ionic domains and permit the foaming process. On the other hand, U.S. Pat. No. 3,322,734 teaches that the acid form of the ionic polymer should not be completely neutralized--preferably the neutralization should be only 80% complete and in no case should exceed 90% of the stoichiometric equivalence. Thus, it is emphasized in U.S. Pat. No. 3,322,734 that incomplete neutralization of the acid moiety is essential in order that the resulting products be fabricated.
The foams described in the present instant invention may be characterized as follows:
(a) The foamed products of the present invention are tough, strong and rubbery at use temperature behaving in many ways like a chemically crosslinked elastomer.
(b) At sufficiently high temperatures the foamed products of the present invention which contain a nonvolatile polar plasticizer can be made to have excellent melt rheological properties so these products, or used or scrap material can be readily melt reprocessed, thereby enabling reuse and refoaming of waste material.
(c) Due to the unique ionic bonding in these systems and the incorporation of a nonvolatile high temperature polar flow improver, it has been found that in an appropriate elevated temperature range these foams have the characteristic of being reformable into different shapes without substantially damaging the foam structure (see e.g., Example 7).
(d) The production of these foams from the bulk material (containing foaming agents, flow improvers and compounding ingredients) exhibits several unusual and important characteristics such as insensitivity of the foam production to temperature changes over a wide temperature region, very rapid foam production because curing is not required, and excellent rheological melt properties.
(e) These thermoplastic elastomer foams can be rapidly produced by either high pressure compression molding or extrusion foaming.
(f) Rubbery foams of good cell uniformity and small cell size can be produced at densities from below 0.1 g/cc up to near bulk density.